Which is correct for threatened (T) species?

$\begin{array}{1 1}(a)\;\text{It is an endangered species like rhinoceros,asiatic lion,which are in danger of extinction}\\(b)\;\text{It is rare species like wild asiatic *** with small population in certain geographical area}\\(c)\;\text{It is vulnerable species like musk deer,blackbuck which are likely to move an endangered category in near future}\\(d)\;\text{It is a species to be conserved to avoid its becoming rare,endangered or vulnerable species}\end{array} $